This invention relates to an improved process for recovery of petroleum from a geological formation containing it. In a particular aspect, this invention relates to an improvement in the process for stimulating petroleum recovery using steam utilization.
The yield of petroleum from a geological formation is usually far from the total petroleum present because once the pressure on the formation is relieved, there is no easy method for recovering the residual petroleum. This residual petroleum is usually of high viscosity and flows to the producing well at a very slow rate--if at all.
It is known to improve petroleum recovery from a geological formation containing it by the process of injecting steam into the formation to heat the petroleum thereby reducing the viscosity and permitting it to flow to the producing well. The steam can be introduced through the production well itself, or it can be injected through one or more injection wells spaced from the production well.
In the former case, the steam is injected for a limited time thereby heating the petroleum and surrounding formation. The steam injection is then stopped and the heated petroleum, having reduced viscosity, flows into the production well.
In the latter case, steam is injected on a continuous basis, more or less, and the steam forms a front which moves through the petroleum-bearing formation and forces the heated oil ahead of it into the production well.
In many petroleum sources, there are acidic components present. Advantage is taken of this in water-flooding processes for petroleum recovery by adding an alkali metal hydroxide, e.g. sodium hydroxide which reacts with the acidic components to form soaps. These soaps reduce the interfacial tension between the oil and water and help to improve the flow of the petroleum. However, this use of alkali metal hydroxides does not lend itself well to the steam injection process because separate means must be provided to inject the caustic into the formation. Also, caustic, being non-volatile, cannot be present in steam, except perhaps as a mist, and remains for the most part in the aqueous phase. As the steam cools, it condenses and water collects in the bottom of the reservoir, taking the alkali hydroxide with it, where it is useless. Fresh steam entering the formation overrides the condensed water but is less effective due to loss of alkali hydroxide. Furthermore, there are also usually present in the formation, compounds of calcium and magnesium which, at the elevated temperatures used, react preferentially with the alkali hydroxide. This is advantageous because it reduces the amount of sodium available to form the petroleum soaps and also generates undesirable, water-insoluble compounds which may tend to plug the formation.
There is, therefore, a need for an improved process for recovering petroleum by the use of steam.